1. Field of the Invention
The present invention relates to a strand pulling-off method for a metal strip, particularly a steel strip, cast in a curved continuous casting plant, wherein the metal strip is initially pulled vertically from a casting mold by a curved entry segment driven at an entry speed, wherein a curved shape is imparted on the metal strip in the curved entry segment so that the metal strip travels in a circular arc, wherein the metal strip, after reaching a horizontal strip travel direction, is straightened by a curved exit segment driven at an exit speed and the metal strip is conveyed out of the curved exit segment, and wherein the metal strip is guided between the curved entry segment and the curved exit segment initially over a reaction roller arranged at the circular arc and then over a guide roller.
The present invention also relates to a curved continuous casting plant for carrying out the method.
2. Description of the Related Art
A strand pulling-off method and a curved continuous casting plant of the above-described type are sold, for example, by SMS Schloemann-Siemag Aktiengesellschaft as so-called CSP plants. Thin slabs having a strip thickness of between 40 mm and 150 mm are cast in such CSP plants.
In curved continuous casting plants, the metal strip is pulled vertically from a casting mold. Subsequently, a curved shape is imparted on the metal strip by a driven curved entry segment or bending driver. After reaching a horizontal strip travel direction, the metal strip is straightened by a driven curved exit segment or pulling-out straightening drive.
In the ideal case, the metal strip extends along an exact circular arc between the two drivers. Among other things, this requires that the two drivers act with exactly the same speed on the metal strip. Differences in the speeds have the result that the metal strip either extends in the shape of a chord between the two drivers or the metal strand sags through. Sagging through of the metal strip can be mechanically counteracted by placing a reaction roller and an additional guide roller against the metal strip from below.
Deviations of the metal strip from its ideal line may result in operating problems and may even lead to interruptions of the casting operation. Accordingly, it is necessary that speed differences are controlled immediately in order to avoid as much as possible any deviations of the metal strip from its ideal line. Moreover, the metal strip should roll with firm contact on at least one roller arranged at the circular arc, so that the stability of the overall travel of the metal strip and, thus, of the entire curved continuous casting plant is increased and the travel is more uniform.
Therefore, it is the primary object of the present invention to provide a strand pulling-off method and a curved continuous casting plant for carrying out the method which make it possible to guide the metal strip with sufficient accuracy on its ideal line and to safely detect any deviations from the ideal line in time for being able to counteract the deviations.
In accordance with the present invention, the guide roller is arranged underneath the circular arc. The method provides for detecting whether the metal strip is in contact with the guide roller or whether the metal strip has lifted off from the guide roller, and the exit speed is raised when the metal strip is in contact with the guide roller and the exit speed is lowered when the metal strip lifts off from the guide roller.
The curved continuous casting plant according to the present invention meets the above object by arranging the guide roller underneath the circular arc and by providing the guide roller with a monitoring device which detects whether the metal strip is in contact with the guide roller or whether the metal strip has lifted off from the guide roller and the monitoring device is in operative connection with a control element which is capable of controlling the exit drive depending on whether the metal strip is in contact with the guide roller or whether the metal strip has lifted off from the guide roller.
Accordingly, when carrying out the strand pulling-off method according to the present invention or when using the curved continuous casting plant according to the present invention, the metal strip is allowed to sag through in a defined manner within a tolerance range. This slight sagging of the metal strip does not have an influence either on the casting process or on the quality of the produced metal strip. However, the decisive advantage achieved with the present invention is that when the exit speed is too high, i.e., when the metal strip has the tendency to assume the shape of a chord, the metal strip is lifted first from the guide roller and would only later lift off from the reaction roller. However, even the lifting off from the guide roller is detected so that the exit speed can be adjusted in time before the metal strip lifts off from the reaction roller.
The guide roller rotates with a circumferential speed. When the metal strip is in contact with the guide roller, the circumferential speed inevitably is the same as the entry speed. Consequently, it is possible to recognize the metal strip as being in contact with the guide roller when the difference between the circumferential speed and the entry speed drops below a predetermined speed barrier, and the metal strip is recognized as being lifted off from the guide roller when the difference between the circumferential speed and the entry speed exceeds the predetermined speed barrier.
Accordingly, the monitoring device may be constructed, for example, as a speed sensor for measuring the circumferential speed of the guide roller.
When the metal strip is in contact with the guide roller, the metal strip exerts a force on the guide roller. Consequently, it is also possible to recognize the metal strip as being in contact with the guide roller when the contact force exceeds a predetermined force barrier, and the metal strip is recognized as being lifted off from the guide roller when the contact force drops below the predetermined force barrier.
Accordingly, the monitoring device may be constructed as a contact force sensor for measuring the contact force with which the metal strip rests against the guide roller.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, specific objects attained by its use, reference should be had to the descriptive matter in which there are described preferred embodiments of the invention.